[ServiceWorker] Security consideration about the `cache` parameter of `Request` (#585)

(Copied from https://github.com/slightlyoff/ServiceWorker/issues/398#issuecomment-66421199)

We should start reviewing each parameter carefully from security point of view.

I'm concerned that fetch() with the cache parameter set to e.g. the only-if-cached option may cause privacy leak. Like CSS :visited (http://dbaron.org/mozilla/visited-privacy), could it be abused to probe if a certain site was visited by the user?

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https://github.com/slightlyoff/ServiceWorker/issues/585